Nuclear Forensics: How Scientists Can Tell Whether North Korea Is Lying About Its Bomb Test

In Tom Clancy's The Sum of All Fears, there's a fascinating moment when nuclear scientists are trying to figure out where the plutonium in a nuclear weapon came from —following a failed attack at the Super Bowl, the government needed to know who was attacking the U.S. By looking at trace contaminants in the non-fissioned plutonium, the scientists could tell not only that it was produced in an American plutonium production reactor, but also that the attack was likely launched by terrorists. Nuclear forensics to the rescue.

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Now let's jump back into non-fiction. This week, North Korea conducted a probably underground nuclear test that, according to North Korea, was a hydrogen bomb. A true H-bomb test is unlikely. More plausible is that the device was "boosted"—that it used a small amount of nuclear fusion to produce copious amounts of neutron radiation that, while contributing little to the explosive yield directly, caused additional fissions that would boost the bomb's yield. Another possibility is that the weapon was simply another pure fission weapon and North Korea is lying through its teeth.

While a true hydrogen bomb seems unlikely, it's a possibility that needs to be taken seriously. Just like in the movies, scientists can use trace evidence to start to figure out the veracity of Kim Jong-Un's claims.

A Quick Refresher on Splitting Atoms

Fission weapons use either uranium (U-235) or plutonium (Pu-239). There are also traces of ordinary U-238 even in weapons-grade uranium, and some types of thermonuclear weapons will also produce energy from U-238 fission. During fission, these atoms release not only a great deal of energy but also a variety of byproducts specific to those atoms. For example, 6.1 percent of the time, fissioning U-235 will produce an atom with an atomic mass of 99; 6.2 percent of the time something with an atomic mass of 137 will be produced. Other fissionable atoms, Pu-239 for example, produce different suites of fission products. The point is that by analyzing the array of radioactive fission products – those that can be captured and analyzed – nuclear forensic specialists can start to figure out if the atoms that fissioned were U-235, Pu-239, or (in some cases) U-238.

Back to North Korea. There are three basic ways it might have made a nuclear weapon: fission-only (using either U-235 or Pu-239), boosted fission, or a full-blown thermonuclear device. There are good reasons to think a true thermonuclear hydrogen bomb is beyond the country's current capability, but it would be nice to be certain of just what kind of weapon it can build.

Analyzing fission products that escape into the atmosphere is where we'll start. Say, for example, that all of the gases and particles collected indicate the fission products are 6.2 percent atoms with an atomic mass of 137, 6.1 percent with 99, and so on. That's the signature of U-235 we mentioned before, and it would be a fairly good indication that the device used U-235. If the math showed the signature of plutonium (6.7 percent with mass 137, 6.2 percent with mass 99), then that would suggest a Pu-239 fission device. Finding just a single fissionable nuclide would suggest there was no thermonuclear yield in North Korea (we'll get to that in a moment), but either could be consistent with either a boosted device or a simple fission bomb. Still, knowing for sure that it wasn't a full-blown thermonuclear device would be huge.

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On the other hand, say that scientists also collect fragments from U-238 fission—more than the 5 to 10 percent that's present in even weapons-grade uranium. This could indicate that the weapon was surrounded by U-238, which is something that can happen in a thermonuclear device. Although U-238 is not as easily fissionable as U-235, it will split under a sufficiently high flux of high-energy neutrons. Detecting evidence of large quantities of U-238 would indicate that a more complex hydrogen bomb had been detonated.

Another indicator of the presence of tritium or deuterium—used either as a fuel or as a boosting agent—would be the discovery of non-fissioned uranium or plutonium contaminated with one of these isotopes of hydrogen (hydrogen, including tritium and deuterium, reacts readily with uranium and plutonium). So if we are able to recover debris from the explosion itself, that debris can be analyzed directly for the boosting agent, or for the hydrogen isotopes that have reacted with the bomb fuel to form uranium or plutonium hydride.

All the Clues

Okay, so isotopic information is one way to investigate whether the North Korean weapon was nuclear, thermonuclear, or tritium-boosted. But this is not the only way.

The yield of the weapon can give a clue. For example, there's a theoretical limit to the yield of a pure fission weapon. That's based on the maximum amount of uranium or plutonium that can be put together without starting a chain reaction while the weapon is in storage. Anything with a higher yield has to be either a boosted or a thermonuclear device. Similarly, there's an upper limit to how powerful a boosted weapon can be. Anything above that limit must be thermonuclear. (There's no theoretical upper limit on the yield of a hydrogen bomb. The largest ever set off was the Soviet Tsar Bomba with a yield of 50 megatons).

Another way would be trying to deduce the amount of energy released by fission compared to the total amount of energy released by a device. For example, we know how much energy is released by a single fissioning atom. So, if we can "count" the number of atoms fissioned (most likely by estimating the amount of radioactivity from fission products), then we can compare the energy released by the weapon to the energy produced by fission. If we calculate that a weapon with a yield of, say, 500 kT produced only 200 kT from fission, then the remaining energy must have come from thermonuclear fusion. That's the hallmark of a hydrogen bomb. This sort of analysis can't tell us about boosting, but it can narrow down the possibilities.

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The more of these clues you can collect, the more you're able to see into North Korea's secrets. Of course, the gold standard would be to collect actual debris from the weapons test. Then, scientists doing nuclear forensics can see the largest number of fission products and can do a better job of determining exactly what the bomb was made of and how it produced its energy. In fact, with enough of the debris it might even be possible to make an educated guess about the materials from which the bomb casing and other components were made, in addition to determining whether the uranium or plutonium fuel came from Russia or China or France or some other nation.

This quality of information is unlikely to be collected at a distance, and Kim-Jong Un isn't going to just let scientists walk into his testing grounds. In addition, every day that passes allows more of the shorter-lived fission products to decay away. The best information is collected right after the detonation and as close to the scene as possible. Most of the information we'll likely get in actuality (barring any spy stuff) will come from aircraft circling North Korea and sniffing for radioactive fission products that have vented to the atmosphere. Although somewhat less satisfying than directly collecting debris from the explosion, collecting this data at a distance might be able to help us sort through the fact and fiction of North Korea's claims.

Andrew Karam is a radiation and nuclear scientist with over 30 years of experience, including 8 years in the U.S. Navy (half of which was spent on a nuclear attack submarine). He has worked on behalf of the International Atomic Energy Agency and traveled to Japan in the aftermath of the Fukushima reactor accident to provide technical assistance to medical and emergency responders. He currently works as a scientist involved in radiological and nuclear emergency preparedness and response.

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